The possibility of using phase change materials (PCMs) in nonvolatile memory cells has recently gained momentum as more is learned about these materials and their integration into integrated circuits. For example, binary and ternary chalcogenide alloys such as antimony telluride and germanium antimony telluride (GST) are showing great promise for use in practical PCM-based memory cells. When incorporated in a memory cell, these materials can be toggled between higher and lower electrical resistance states by applying a pulse of electrical current (“switching current pulse”) to the memory cell. Subsequently, after writing to a memory cell in this way, the electrical resistance state of the given memory cell can be determined (i.e., read) by applying a low magnitude sensing voltage to the material in order to determine its electrical resistance state.
A phase change feature in a typical PCM-based memory cell is disposed between two conductive electrodes and is otherwise surrounded by a dielectric material such as silicon dioxide. The two electrodes are operative to cause an applied switching current pulse to flow through the phase change feature. This switching current pulse, in turn, results in ohmic heating in the PCM and induces some or all of the PCM to change phase. The dielectric portion of the memory cell, on the other hand, serves several different purposes within the memory cell. First, it electrically isolates the phase change feature from other nearby electrically-active features. In addition, it thermally insulates the phase change feature when switching phases, thereby reducing the magnitude of the switching current pulse needed to write to the memory cell. Finally, the dielectric portion protects the phase change feature from contamination from other nearby materials and from those processing steps occurring subsequent to forming the feature.
Nevertheless, it has recently been observed that seams and other similar defects in the dielectric portions of PCM-based memory cells may induce defects in the phase change features of these memory cells, causing the memory cells to not function properly or to suffer from poor reliability. Accordingly, there is a need for new PCM-based memory cell designs that reduce the deleterious effects of seams in the dielectric portions of the memory cells.